We usually play on a top vs bottom map. Coop is kinda bad because if you don't have even teams, the combined research advantage is pretty insurmountable. You also can't have AI on your team, as it will constantly declare war or sue for peace when you're fighting and is generally just bad.

I was thinking like a us vs all the AI on a large map (getting real 2v2 multiplayer actually seems sort of lame, because almost the whole point of Civ is that humans can exploit things by optimizing in ways the AI can't or won't), but I don't know that you can do that without everybody just ganging up on you?

You absolutely can, there's even a map set for it. I think either Ring or Donut is considered best (can't remember off the top), but if you crank the difficulty, something like Small Continents work well. Enormous Pangaea works, but can take ages to get going on a map that big. Anyway, yeah, there are a bunch of maps designed to stop the gang-up thing and a few of them have team play in mind.

Latest few patches have made multiplayer more playable. I'm in the middle of a pretty lengthy game with my brother, entering the era of airplanes and submarines, and we still haven't see that loading screen pop up yet.

So I've been messing with Multiplayer Mod Pack DLC Hacking. It definitely seems to add a couple desyncs (I added a shit-ton of extra civs, and Enhanced UI which is totally awesome btw - makes it a bit more visually cluttered, but a lot less clicks to do things and a lot more information provided.) and one of the civs I added is glitched in multiplayer. Adds a bit of fun, although I'm not sure just how balanced the added civs are - my best friend and I are playing co-op vs AIs, playing Scotland teamed up with the Zulus, and once he finishes building the Brandenburg Gate and a Military Academy, we can train Impi that start with the full Buffalo set, plus Drill 1-3. Once we get ideologies and he gets a Level 2 policy, we'd be able to start Impi with full Buffalo + Drill 1-3 + Blitz, or Gunpowder and on units with Drill 1-3 + Blitz + March + Siege. He's also currently farming Hitler('s last remaining city...) for EXP.